1. Field of the Invention
The present invention relates to the anti-oxidation protection of a material of which at least a surface is made of a ceramic formed from a silicon compound.
In particular, the present invention is aimed at composite materials, and more particularly thermostructural materials, that contain an oxidizable layer.
Thermostructural composite materials are characterized by good mechanical properties and their ability to retain these properties up to high temperatures. They are made from a refractory fibrous reinforcement that is densified by a matrix that is likewise refractory. The refractory materials making up the fibers and the matrix can be carbon (C) or a ceramic, such as silicon carbide (SiC). The resulting composite materials can be of the following type: C/C, C/SiC (carbon fiber reinforcement and SiC matrix), SiC/SiC, or C/C-SiC (in which the matrix has respective carbon and SiC phases). Their mechanical characteristics can be enhanced by forming a pyrolytic carbon or boron nitride intermediate layer, also known as an interphase, between the fibers and the matrix.
It is essential that these materials be provided with an anti-oxidation protection, otherwise they would rapidly deteriorate when exposed to an oxidizing atmosphere at high temperature.
2. Prior Art
Silicon carbide (SiC) is widely used for forming an anti-oxidation layer. The SiC layer can be obtained by different processes, such as chemical vapor infiltration or deposition, pack cementation, or silicon inducement of a composite material. With composite materials having an SiC matrix, the SiC coating is provided by the outer layer of the matrix.
Usually, the protective action of the SiC coating is completed by an external surface coating. The latter is advantageously made of silica (SiO.sub.2) glass that forms a barrier against oxygen diffusion. The coating also possesses healing properties, since it constitutes a vitreous layer whose viscosity is such to fill in any cracks appearing in the SiC coating when the material is at its high service temperature. This vitreous layer thus helps improve the crack resistance of the material protects. The viscosity of this viscous layer can be controlled by the introduction of different additives, e.g. to adapt the viscosity to the temperature range in which the material is to be exposed in use. It should be noted that even in the absence of an external vitreous layer, the passive oxidation of the SiC leads to the formation of an external SiO.sub.2 oxide layer.
The anti-oxidation protection afforded by an SiC coating, with or without an SiO.sub.2 based vitreous layer, is satisfactory for encountering conditions involving a passive oxidation of the SiC.
However, this is no longer so when active oxidation conditions are encountered, that is at temperature and pressure conditions under which the oxidation causes the formation of volatile species (SiO) that anihilate the protective effect. Indeed, such conditions lead to a deterioration of the SiC coating, which rapidly gives the oxidizing species easy access to the oxidizable layer of the composite material.
The formation of SiO gas can result from a chemical reaction between SiC and an oxidizing species from the environing medium, such as water or oxygen, or from a chemical reaction between SiC and an SiO.sub.2 layer.
The transition between passive and active oxidation of SiC occurs at lower temperatures as pressure diminishes. Consequently, active oxidation of SiC can take place under certain conditions of use of the thermostructural materials, for instance when they are employed as heat shielding elements in space vehicles that experience considerable heat upon re-entry into the upper atmosphere.
Several solutions have been put forward to solve the problem caused by active oxidation of SiC at high temperature. In particular, reference can be made to documents EP-0 310 043 and FR-A-2 635 773, as well as a paper entitled "Ceramic Coatings for Carbon Materials" by J. E. Sheehan, published in Proceedings on Material Technology, May 1987.
However, none of these documents discloses the presence of an SiO.sub.2 based external vitreous layer-if anything, they advise against it.
The above discussion concerning SiC applies equally to other ceramic materials formed from a silicon compound, and in particular to silicon nitride (Si.sub.3 N.sub.4). Si.sub.3 N.sub.4 does indeed have similar characteristics to those of SiC, and is known both as a material susceptible of forming the matrix of a ceramic matrix composite material, and as a material that can constitute an anti-oxidation protective coating. Under passive oxidation conditions, there is formed an SiO.sub.2 layer on an Si.sub.3 N.sub.4, whereas under active oxidation conditions, Si.sub.3 N.sub.4 reacts in the same way as SiC.